FISICA AMBIENTALE 1
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Transcript of FISICA AMBIENTALE 1
FISICA AMBIENTALE 1
Radioattività:misure 2
Lezioni 29-30
Marie Curie
RIVELATORI A STATO SOLIDOMeasurement of radiation energy is achieved by mechanism
of ionization of fixed atoms in a solid semiconductor.The e- exist in definite energy bands. Radiation can extract an e- to the conduction band, leaving a hole in the valence band.
Electron-hole pair can be compared with ion pairs in a gas
BAND STRUCTURE FOR ELECTRON ENERGIES IN INSULATORS AND SEMICONDUCTORS
INSULATOR SEMICONDUCTOR
Valence band
Valence band
Eg > 1eVEg > 5eV
Conduction band Conduction
bandElectronenergy
Cutaway view of a transmission mount, in which both surfaces of the silicon wafer are accessible. The coaxial connector is placed at the edge of the ceramic ridge
ABCD
E
SILICON JUNCTION DETECTOR IN CROSS SECTIONAL VIEW
Surface barrier mount with coaxial connector (E) at rear. The silicon wafer (C) is mounted in a ceramic ring (A) with electrical contact made between either side of the junction and opposite metalized surfaces.The front surface is connected to the center conductor of the coaxial connector (E).
Energy levels of an organic molecule with -electron structure
Photo-electrontrajectory
Basic elements of a photomultiplier tube
EFFETTI FOTOGRAFICIIonizing radiation affects photographic film as visible light does.
With the aid of special film holders incorporating filters, the photographic film enables information on the type and energy of radiation
DOSE-DENSITY CURVE
After processing, the film is read by passing a beam of light through it and measuring the optical density which is
converted to radiation dose by means of a calibration curve
ELETTRONICA
d.c. amplifier – negative feedback
Amplification of a very low current to a high enough value to operate a conventional ammeter. A negative feedback reduces instability due to temperature fluctuation caused by very high gains
A discriminator rejects all pulses below a certain level which is set by applying a discriminator bias voltage
FUNCTION OF A DISCRIMINATOR
PLATEAU FOR G-M COUNTERIn order to set up the equipment, a small source is placed near the detector. The graph obtained by plotting the count rate is called plateau, relatively independent on the applied V over a certain range.
COUNTING EQUIPMENT – SCHEMATIC DIAGRAM
I60Co –ray spectrum
BASIC RATIMETER CIRCUIT
C
Reservoircapacitor
RPulses in
Each pulse feeds a charge into the capacitor C, which then slowly discharges through the resistance R, giving a reading on the meter.
Relationship between half-value thickness (g/cm2) and –ray maximum energy
DECAY CURVE OF MIXED Cl-38 AND Na-24 SAMPLE